<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>

    <style>
        .box {
            width: 200px;
            height: 200px;
            border: 1px solid #000;
        }

        .red {
            background-color: #f00;
        }
    </style>
</head>
<body>
    <div id="app">
        <button @click="isRed = !isRed">切换</button>

        <!-- 不加冒号代表写死的类，加冒号代表动态类 -->
        <div class="box" :class="{ red: isRed }"></div>
    </div>

    <script src="./vue.js"></script>
    <script>
        new Vue({
            el: '#app',
            data: {
                isRed: false
            }
        })
    </script>
</body>
</html>